JP-B 03-81119 discloses a feed drive apparatus provided with a drive leg having an extension/contraction deforming part and a shear deforming part, wherein the extension/contraction deforming part is extended so that the tip of a drive leg comes in contact with the moving element or with a fixed side and the shear deforming part deforms in the drive direction or the reverse direction to drive a moving element.
This feed drive apparatus drives the moving element using an approach similar to the principle of human ambulation. When the tip of the drive leg is brought in contact with the moving element or the fixed side to cause a shear deformation, the moving element is driven by the frictional force generated between the tip of the drive leg, and the moving element or the fixed side.
FIG. 21 shows the voltage applied to the extension/contraction deforming part and shear deforming part of the feed drive apparatus which has two types of drive legs, i.e., an A leg and a B leg, respectively. Herein, a square wave-like drive voltage is applied to the extension/contraction deforming part and the shear deforming part, respectively, and the voltage is controlled by a cycle of 6 phases with a phase difference of 60° which respectively overlap by 30° at a time. However, the piezoelectric elements forming the extension/contraction deforming part and shear deforming part act as capacitors, so the waveform is actually a trapezoidal wave-like voltage.
In such a feed drive apparatus, to increase the drive speed, the drive voltage frequency is not changed, and the voltage applied to the shear deforming part is increased. However, if the voltage is increased, the piezoelectric element forming the shear deforming part acts as a capacitor, and the shear deformation time will become longer due to the time constant. Thus, a shear deformation occurs even when the drive leg in which this shear deforming part is provided does not touch the moving element, and a shear deformation occurs when the other drive leg is not completely separated from the moving element. This prevents a high drive speed from being attained.
When a drive voltage as shown in FIG. 21 is applied to a drive leg, the charge and discharge time constants of the extension/contraction deforming part or the shear deforming part comprising piezoelectric elements differ depending on whether the voltage is positive or negative, and as this is a sequence, the extension time and contraction time are not necessarily equal. FIG. 22 shows the voltage changes applying to these deforming parts. If the voltage applied to the deforming parts is integrated, the integral value, i.e., the average of the voltage, will be negative. This is one factor leading to depolarization of the piezoelectric element, and a shorter lifetime of the piezoelectric element.